Make Intel's Inference Engine backend is default if no preferable backend is specified.

modules/dnn/src/layers/fully_connected_layer.cpp

@@ -128,7 +128,7 @@ public:
 
     virtual bool supportBackend(int backendId) CV_OVERRIDE
     {
-        return backendId == DNN_BACKEND_DEFAULT ||
+        return backendId == DNN_BACKEND_OPENCV ||
                backendId == DNN_BACKEND_HALIDE && haveHalide() && axis == 1 ||
                backendId == DNN_BACKEND_INFERENCE_ENGINE && haveInfEngine() && axis == 1;
     }